[ this ]              is null     (       --> [ )

  [ 0 unrot
    witheach
    [ dip
        [ behead rot ]
      * + swap ]
    drop ]             is [a]->     (   [ [ --> n )

  [ [] unrot
    reverse
    witheach
      [ /mod rot
        join swap ]
    drop ]             is ->[a]     (   n [ --> [ )